In a conventional steering control system for a vehicle especially for an automobile, an operation angle of a steering wheel (i.e. a steering wheel operation angle) has been publicly known to be transmitted to a vehicle wheel to be steered without being varied. That is, the steering wheel operation angle is always transferred at one for one rate for a vehicle wheel steering angle. However, recent developments have lead to the vehicle steering control system mounting a variable steering angle conversion ratio mechanism by which a conversion ratio for converting the steering wheel operation angle to the vehicle wheel steering angle (hereinafter, referred to as a steering angle conversion ratio) is varied in accordance with vehicle driving conditions such as a vehicle speed. At a time of vehicle high-speed travel, it is preferable to set the steering angle conversion ratio to be a relatively small ratio. In this case, the steering angle can be prevented from being rapidly increased in response to increase of the steering wheel operation angle, thereby enabling to stabilize the vehicle high-speed travel. On the other hand, at a time of vehicle low-speed travel, it is preferable to set the steering angle conversion ratio to be a relatively large ratio. In this case, the vehicle wheel can be steered to the maximum steering extent possible with the small steering wheel operation angle. That is, the driver does not have to operate the steering wheel many times so as to steer the vehicle wheel to the maximum steering extent possible. Therefore, it makes easier to perform driving performance, which requires the vehicle wheel to be steered at a relatively large steering angle, such as parking to a garage, parallel parking, and pulling over to the kerb.
This type of variable steering angle conversion ratio mechanism has been disclosed in a Japanese Patent Laid-Open Publication published as No. 1999-834604. Disclosed above is a geared transmitting unit which connects a shaft connected to the steering wheel and a vehicle wheel steering shaft with a variable gear ratio. However, a gear ratio changing mechanism of the geared transmitting unit may become complicated.
In another Japanese Patent Laid-Open Publication published as No. 1999-334628, disclosed is a vehicle steering control unit with a variable steering angle conversion ratio mechanism in which the vehicle wheel steering shaft is driven for its rotation by an electric motor. More particularly, a target vehicle wheel steering angle is computed based upon the steering wheel operation angle detected by an angle detecting unit and the steering angle conversion ratio determined in accordance with the vehicle driving conditions. A motor rotates the vehicle wheel steering shaft, which is mechanically disconnected from the shaft connected to the steering wheel, so as to steer the vehicle wheel at the target vehicle wheel steering angle.
According to the vehicle steering control unit with the variable steering angle conversion ratio mechanism, it is preferable that the vehicle wheel steering shaft rotates in compliance with the rotation of the shaft connected to the steering wheel. Therefore, a rotational speed of an electric motor shaft has been adjusted by a pulse width modulation control (hereinafter, referred to as a PWM control) so as to decrease or vanish an angle deviation of a steering shaft angle position of the vehicle wheel steering shaft from a target angle position thereof.
The electric motor may be frequently driven for rotate the vehicle wheel steering shaft in response to the frequent operation of the steering wheel. Therefore, the electric motor is demanded to operate stably over a long period of time, which may lead to enhancement of vehicle durability. For example, the operating performance of the motor may deteriorate after being applied with excessive electric current over a long period of time. In light of foregoing, the operation of the motor can be appropriately adjusted by restraining the electric current to be supplied to the motor. An electric current sensor can be used for monitoring a value of the electric current being supplied to the electric motor.
However, in the motor applied with the PWM control, large flywheel current may occur due to inductive load characteristics at a time of switching on/off an electric power source such that the electric current being supplied to the motor may not be detected precisely.
A need thus exists for providing a vehicle steering control system capable of detecting the electric current supplied to the motor under the PWM control.